Godsmack may be big enough to fill an amphitheater. When Disturbed/Godsmack headlined Mayhem last year the place was more packed for Godsmack and people were leaving for Disturbed.

That doesn't necessarily mean all the people watching Godsmack were there to see Godsmack. Many of those people could have just been waiting to watch Disturbed. I'm sure if you switched Disturbed and Godsmack in the lineup you'd see a similar result.

That being said, I do believe Godsmack are big enough to fill an amphitheatre. Maybe not sell it out, but these tours usually don't actually "sell out" more than just fill 2/3s of it and then just sell the rest of the tickets at a discount price or the organizers buy the remaining seats so it can be billed as "sold out"